LHC the Physics

17 Jun 1994

Physics for the 21st Century

Geneva, 17 June 1994. On 24 June 1994, delegates representing CERN1's 19 European Member States will decide whether to approve the construction of the Large Hadron Collider (LHC), a huge scientific instrument which will propel particle physics research way into the 21st century.

The LHC, a particle accelerator built of more than a thousand superconducting magnets each 13 metres long, will be installed in CERN's existing 27-kilometre circular tunnel built for the LEP electron-positron collider. These powerful magnets will hold counter-rotating beams of protons on a steady course around the ring as superconducting accelerating cavities 'kick' them almost to the speed of light at energies higher than have ever been reached. When these proton beams collide at fixed crossing points their combined energy of motion (14 Tera-electron volts or TeV) will produce an intense micro-fireball which will shoot out hundreds of new particles. These flashes of energy will probe the interactions between the tiny quark constituents hidden deep inside the colliding protons and reveal how Nature works at the most fundamental levels.

Over the last two decades physicists have painstakingly pieced together a consistent picture of the subnuclear world known as the 'Standard Model'. All experimental results have so far confirmed this picture. However, the 'Standard Model' has too many unknown quantities to be the ultimate theory. The LHC has enormous discovery potential, as it will be penetrating a totally unexplored energy region where physicists are convinced that new behaviour must be seen and the Standard Model will finally break down.

The most important enigma facing particle physicists today concerns mass. While the concept of mass may well appear so fundamental that it should be beyond question, particle physics has thrown up many puzzling questions about the nature of mass, questions which are not answered by the Standard Model. For instance, unlike the chemical elements, the fundamental particles in physics show no regularity in their masses. The tau lepton is some 17 times heavier than the muon, and 3491 times heavier than the electron. Other, similarly mysterious ratios are found among quarks, while neutrinos may even be massless. The Standard Model is unable to explain these masses, and a major task for particle physicists is to uncover the origin of mass. Is there some underlying reason why quarks and leptons have their particular masses? Why do these masses vary so much, and why do some particles have mass while others are massless?

The present 'answer' to these questions is provided by the subtle 'Higgs' mechanism which suggests that particles acquire mass by interacting with a force field, the Higgs field, which is everywhere present. The discovery of an associated particle or particles, the Higgs boson(s), would be evidence for this field. No sign of Higgs particles has yet been seen, but calculations based on the Standard Model suggest something has to show up when quark energies reach about 1,000 GeV (1 Tera-electronvolt, or TeV). This is exactly the energy range which the LHC has been designed to explore and whatever the Higgs mechanism is, the LHC will surely reveal it, opening up an entirely new era in our understanding of Nature.

Finding the solution to the mystery of mass is not the only discovery within the LHC's reach. Perhaps the most dramatic is a question which has been posed by cosmologists rather than particle physicists - 'what does space contain?' Astronomical observations show that there is more matter in existence than has yet been seen. Shining objects such as the Earth, all of the planets and all of the stars only add up to about one tenth of existing matter. The other nine-tenths we call 'Dark Matter'. One explanation for Dark Matter envisages the existence of stable, as yet undiscovered, particles and the most recent results from LEP suggest that a new family of particles may exist at precisely the energy which the LHC will explore. The discovery of these new, 'supersymmetric' particles could explain what the vast majority of our Universe is made of.

Another fundamental question posed by cosmologists is "Why does the matter in the Universe exist?" At the time of the Big Bang, matter and antimatter should have been produced in identical amounts. The Universe should then have had a very short life, because these two different sorts of particles annihilate each other. Nonetheless, the Universe has survived as predominantly matter. In the 1960s, Soviet theorist Andrei Sakharov formulated an explanation for the dominance of matter over antimatter, based on a small asymmetry in the behaviour of matter and anti-matter particles. In 1973, Japanese theoreticians showed that a Universe made up of three families of quarks and leptons could satisfy Sakharov's requirements. The subsequent confirmation at CERN of the existence of exactly three matter particle families suggests that this theory may be the right approach to explaining the present state of the Universe. There is still an enormous amount of work to be done on this subject and the LHC will be the perfect tool to allow physicists to examine this asymmetry of matter and antimatter by detailed studies of the behaviour of the quark known as the beauty quark.

The LHC is the Swiss Army Knife of particle accelerators. Its versatility allows it to be turned to several different tasks, primarily because it occupies the same site as other CERN colliders and particle sources. Accelerated heavy ions such as lead nuclei may be produced in CERN's accelerator complex and injected into the LHC. Collisions between these chunks of matter produce very large concentrations of energy, allowing studies of the "quark-gluon plasma", a state of matter which may have existed shortly after the Big Bang and in the cores of collapsed stars. Another "spin off" could be to combine the LHC and LEP (sharing the same tunnel), to produce electron/proton collisions of around 1.5 TeV energy.

CERN's LHC is the supermachine for the 21st century. The decision of CERN's Council on 24 June is eagerly awaited as it will launch the next phase of our quest to understand the Universe which surrounds us.

Footnote(s)

1. CERN, the European Laboratory for Particle Physics, has its headquarters in Geneva. At present, its Member States are Austria, Belgium, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, the Slovak Republic, Spain, Sweden, Switzerland and the United Kingdom. Israel, the Russian Federation, Turkey, Yugoslavia (status suspended after UN embargo, June 1992), the European Commission and Unesco have observer status.